A communication technology using a high frequency (about 100 MHz to 200 GHz) including microwave (about 1 GHz to 200 GHz) has become important in recent years, and the technology is applied widely to cellular phones, wireless communications, satellite broadcasting, in-vehicle radars, etc.
Currently, a phase-locked oscillation circuit is widely used for high-frequency oscillation. FIG. 15 shows a conventional phase-locked oscillation circuit. Briefly speaking, the phase-locked oscillation circuit of FIG. 15 is configured of a VCO (Voltage-Controlled Oscillator) 20 and a phase-locked loop circuit 2 (which will hereinafter be referred to as “PLL circuit”). The VCO has a disadvantage that is an unstable oscillating frequency fout (temporal change in a phase). Because of the unstable oscillating frequency, an oscillation spectrum is not narrowed and sharped but wide. That is, a peak of the spectrum is broad. Therefore, the PLL circuit 2 is provided in order to narrow the peak width, in other words, to achieve the phase synchronization. Basically, the PLL circuit 2 is configured of a reference signal source 1 that outputs a reference signal with a frequency fref, a phase comparator 3, a loop filter 4, and a frequency divider 9. An output from the phase comparator of the PLL circuit (which will hereinafter be referred to as “phase error signal”) achieves the phase synchronization.
However, the VCO includes a coil and a capacitor as its components when the oscillating frequency is low while including a dielectric resonator as its component when the oscillating frequency is high, and therefore, it is difficult to downsize the VCO (into an IC package). Therefore, in order to achieve the downsizing, it has been suggested that a magnetoresistive element (which will hereinafter be referred to as “MR element”) is configured to be a high-frequency oscillation source. For example, MR elements described in Japanese Patent Application Laid-open Publication No. 2006-295908 and H. Kubota, K. Yakushiji, A. Fukushima, S. Tamaru, M. Konoto, T. Nozaki, S. Ishibashi, T. Saruya, S. Yuasa, T. Taniguchi, H. Arai, and H. Imamura: Applied Physics Express 6 (2013) 103003 and H. Maehara, H. Kubota, Y. Suzuki, T. Seki, K. Nishimura, Y. Nagamine, K. Tsunekawa, A. Fukushima, A. M. Deac, K. Ando and S. Yuasa: Applied Physics Express 6 (2013) 113005 match the suggestion. The MR element can be manufactured by a semiconductor manufacturing process. Therefore, a small-sized element having a size of “(about 30 nm to 300 nm in diameter)×(about 30 nm to 100 nm in height)” can be manufactured.
Basically, the MR element has a magnetization flexible layer (which will hereinafter be referred to simply as “free layer”), a non-magnetic layer, and a magnetization fixed layer (which will hereinafter be referred to simply as “fixed layer”). When a direct current is caused to flow in a direction from the free layer to the fixed layer or in a direction reverse to that direction, electron spins in the free layer are excited by a spin torque of the direct current, and the electrons are oscillated at a frequency that is unique to the free layer, so that a high-frequency wave (microwave) is oscillated. However, this high-frequency wave oscillated by the MR element has a first problem that is the unstable oscillating frequency fout (large peak width) as similar to the case of the VCO.